Learn about this topic in these articles:

glycolysis

...produced in step [6] reacts with ADP in a reaction catalyzed by phosphoglycerate kinase, with the result that one of the two phosphoryl groups is transferred to ADP to form ATP and
3-phosphoglycerate. This reaction [7] is highly exergonic (
i.e., it proceeds with a loss of free energy); as a result, the oxidation of glyceraldehyde 3-phosphate, step [6], is irreversible....